The present invention relates to the field of electronic image sensor circuits. In particular, the present invention relates to CMOS photo sensor design and wafer processing of photo sensors within the framework of a CMOS process.
Photodetectors are integrated into semiconductor circuitry, for specialized processors that can drive a wide variety of opto-electronic devices. Important photodetector performance criteria include:
a) efficiency and wavelength dependence of electron-hole pair generation from incident photons.
b) efficiency in transporting electron-hole pairs to the collection region where current is measured.
c) generation of electron-hole pairs in the device in the absence of light (xe2x80x9cdark currentxe2x80x9d noise).
Energy absorbed in a semiconductor can excite a valence band electron into the conduction band. The excitation energy required is called the bandgap energy. When an electron is excited to the conduction band, a hole is left behind in the valence band. This process is commonly called electron-hole pair generation.
The generation of an electron-hole pair temporarily increases the charge carrier concentration in the semiconductor beyond the steady-state value defined by the mass action law (np=ni2). (xe2x80x9cnxe2x80x9d) is the free electron concentration. (xe2x80x9cpwxe2x80x9d) is the free hole concentration. ni=pi are the natural intrinsic carrier concentrations in pure defect free silicon at a given temperature. Electron-hole pair generation increases both n-type and ptype carrier concentrations by the same amount, but in a doped semiconductor, the minority carrier concentration is often significantly increased while the percentage increase in concentration of abundant majority carriers is very small. Excess minority carriers (electrons in a p-type semiconductor or holes in an n-type semiconductor) can then survive only a short time before recombining with abundant majority carriers to reestablish the steady-state carrier concentration. The average time that a minority carrier survives is inversely proportional to the majority doping concentration in the material. In a doped semiconductor, as long as the carrier generation level is low enough that the minority carrier concentration does not approach that of the majority type, carrier generation is proportional to the energy absorbed.
A p-n junction diode can be used to collect minority carriers before they recombine, producing a current proportional to the amount of energy absorbed by the semiconductor. By applying a reverse bias (positive voltage on the n-type terminal and relative negative voltage on the p-type terminal), an electric field is established across the depletion layer of the junction. Excited electrons in the p-side that diffuse to the depletion layer boundary are swept to the n-side due to the electric field, and excited holes in the n-side that reach the boundary are drawn across to the p-side. This flow of carriers from the minority side to the majority side of the p-n junction is measured as a reverse current in the diode and is proportional to the energy absorbed near the p-n junction. Collection and measurement of the electron-hole pairs generated in such a structure thus requires a junction proximal to the point of carrier generation and relatively low doping to allow diffusional transport to the junction boundary before recombination.
Silicon absorbs light energy from the visible and near infrared spectrum thereby generating electron-hole pairs. The light-generated electron-hole pairs are measured for photo detection. Short wavelength (high energy) blue light is most readily absorbed (90% absorption at xcx9c1 xcexcm depth). Longer wavelength red light travels deeper before absorption (90% absorption at xcx9c10 xcexcm depth).
Collecting the charge created from light absorption requires that the charge be generated within or near the depletion region that surrounds the p-n junction boundary. Electron-hole pairs generated within this region are immediately separated due to the reverse bias field, and contribute to the collected charge. The charge generated outside this region can also contribute to the collected charge if the minority carriers travel to the boundary of the junction. Because the depletion region width (a few tenths of a micrometer) is much smaller than the depth of light collection, the primary means of photocurrent collection is through transport of carriers to the depletion boundary.
Incident light energy is normally measured indirectly from the amount of electron hole pairs generated by light absorption. An ideal photodetector would measure only the light energy absorbed in the semiconductor. However, there are sources other than light that contribute to electron-hole pair generation.
For example, thermally generated electron-hole pairs can significantly add to the reverse current in the p-n junction. Thermally generated carriers are often concentrated at the surface of the semiconductor due to the discontinuity in the lattice structure. The discontinuity results in energy states within the forbidden energy gap that enhance carrier generation. Mechanical stress in the semiconductor die also enhances carrier generation. In a CMOS fabrication process that uses LOCOS isolation, stress is concentrated at the tip of the SiO2 isolation structure known as a xe2x80x9cbird""s beakxe2x80x9d. Carriers generated by mechanical stress and thermal excitation are collected by a photo detector just as those generated by light.
The process of photo detection is degraded by these non light-based electron-hole pair generation sources because it takes some amount of light-generated current to produce a signal level large enough to overcome the dark current. It would therefore be desirable to minimize the effect of these other electron-hole pair generation sources.
It is the object of the present invention to produce more efficient photo sensors based on integrated circuitry optimized within the CMOS process framework. The present invention mitigates the nominal degradation of integrated photo sensor performance by implementing specific doping profiles and design geometries. For example, some embodiments of the present invention use low doping concentrations to create a p-n junction that reduces the number of majority carriers available for recombination Some embodiments grade the doping profile in the p-type and/or n-type terminals of the diode in a manner to create a built-in electric field throughout the region where light absorption occurs. This causes minority carrier drift away from the more highly doped region. The minority carrier drift mechanism is designed to direct minority carriers towards the p-n diode junction. However, the designs of the present invention do not significantly alter the base process required to fabricate the rest of the integrated circuit (IC) device. The designs of the present invention increase the output signal by maximizing the conversion of light energy to output current. Furthermore, the present invention decreases dark current by minimizing the conversion of other energy sources to output current. The present invention thereby maximizes the photo sensor dynamic range.
Other objects, features, and advantages of present invention will be apparent from the accompanying drawings and from the following detailed description.